1. Field of the Invention
The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to flexible and/or elastic MRI antenna arrays for use in receiving MRI signals.
2. Description of Related Art
A. Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) refers generally to a form of clinical imaging based upon the principles of nuclear magnetic resonance (NMR). Any nucleus which possesses a magnetic moment will attempt to align itself with the direction of a magnetic field, the quantum alignment being dependent, among other things, upon the strength of the magnetic field and the magnetic moment. In MRI, a uniform magnetic field B0 is applied to an object to be imaged; hence creating a net alignment of the object's nuclei possessing magnetic moments. If the static field B0 is designated as aligned with the z-axis of a Cartesian coordinate system, the origin of which is approximately centered within the imaged object, the nuclei which possess magnetic moments precess about the z-axis at their Larmor frequencies according to their gyromagnetic ratio and the strength of the magnetic field.
Water, because of its relative abundance in biological tissues and its relatively strong net magnetic moment Mz created when placed within a strong magnetic field, is of principle concern in MR imaging. Subjecting human tissues to a uniform magnetic field will create such a net magnetic moment from the typically random order of nuclear precession about the z-axis. In a MR imaging sequence, a radio frequency (RF) excitation signal, centered at the Larmor frequency, irradiates the tissue with a vector polarization which is orthogonal to the polarization of B0. Continuing our Cartesian coordinate example, the static field is labeled Bz while the perpendicular excitation field B1 is labeled Bxy. Bxy is of sufficient amplitude and duration in time, or of sufficient power to nutate (or tip) the net magnetic moment into the transverse (x-y) plane giving rise to Mxy. This transverse magnetic moment begins to collapse and re-align with the static magnetic field immediately after termination of the excitation field B1. Energy gained during the excitation cycle is lost by the nuclei as they re-align themselves with B0 during the collapse of the rotating transverse magnetic moment Mxy.
The energy is propagated as an electromagnetic wave which induces a sinusoidal signal voltage across discontinuities in closed-loop receiving coils, this signal voltage being inversely and non-linearly proportional to the distance between the target voxel and coil element. This represents the NMR signal which is sensed by the RF coil and recorded by the MRI system. A slice image is derived from the reconstruction of these spatially-encoded signals using well known digital image processing techniques.
B. Local Coils and Arrays
The diagnostic quality or resolution of the image is dependent, in part, upon the sensitivity and homogeneity of the receiving coil to the weak NMR signal. RF coils, described as “local coils” may be described as resonant antennas, in part, because of their property of signal sensitivity being inversely related to the distance from the source. For this reason, it is important to place the coils as close to the anatomical region-of-interest (ROI) as possible.
Whereas “whole body” MRI scanners are sufficiently large to receive and image any portion of the entire human body, local coils are smaller and therefore electromagnetically couple to less tissue. Coupling to less tissue gives rise to coupling to less “noise” or unwanted biologically or thermally generated random signals which superimpose upon the desired MR signal. The local coils may be of higher quality factor (Q) than the body coils due to their smaller size. For all of these reasons, local coils typically yield a higher signal-to-noise (S/N) ratio than that obtainable using the larger whole body antenna. The larger antenna is commonly used to produce the highly homogenous or uniform excitation field throughout the ROI, whereas the local coil is placed near the immediate area of interest to receive the NMR signal. The importance of accurate positioning leads to the development of local coils which conform to the anatomy of interest, yet function to permit ease of use.
While the smaller local coil's size works to an advantage in obtaining a higher S/N ratio, this reduced size also presents a disadvantage for imaging deep-seated tissues. Typically, the single-conductor coil diameter which yields the optimal S/N ratio at a depth ‘d’ is a coil of diameter ‘d’; hence, larger diameter single-conductor coils are required to image regions in the abdomen and chest of human patients.
The S/N ratio of the NMR signal may be further increased by orienting two coils, or coil pairs about the imaged object so that each detects RF energy along one of a pair of mutually perpendicular axes. This technique is generally known as quadrature detection and the signals collected are termed quadrature signals.
The outputs of the quadrature coils are combined so as to increase the strength of the received signal according to the simple sum of the output signals from the coils. The strength of the noise component of these signals, however, will increase only according to the square root of the sum of the squares of the uncorrelated noise components. As a result, the net S/N ratio of the combined quadrature signals increases by approximately √2 over the S/N ratio of the individual coils.
The quadrature orientation of the two coils introduces a 90° phase difference between the NMR signals detected by these coils. Therefore, combining the outputs from the two quadrature coils to achieve the above described signal-to-noise ratio improvements requires that one signal be shifted to have the same phase as the other signal so that the amplitudes of the signals simply add in phase.
The approximate net gain of √2 in S/N ratio is achievable primarily due to the lack of inductive coupling between the coil pairs. This ensures that only the uncorrelated noise components add, in lieu of both the uncorrelated and correlated noise components, to reduce the effective S/N ratio. Inductive isolation is achieved by geometrically orienting the coil conductors such that the mutual inductance is minimized between the coil pairs according to the following:
  M  =            1              2        ⁢        π              ⁢          ∫                                                  I              1                        ⁡                          (                                                ⅆ                                      l                    1                                                  _                            )                                ·                                    I              2                        ⁡                          (                                                ⅆ                                      l                                          2                      ⁢                                                                                                                                          _                            )                                                                                    (                                                r                  1                                _                            )                        -                          (                                                r                  2                                _                            )                                                    
where M represents the mutual inductance between coils 1 and 2 and the vector components dl1 and dl2 represent segments of coils 1 and 2 with current amplitudes I1 and I2. The denominator represents the magnitude difference of the position vectors of each dl segment. The condition wherein M is approximately zero with respect to the individual self inductances of coils 1 and 2 is known as inductive isolation between the coils.
C. Multiple Channel Receiver/Coil Systems
A method of increasing the S/N ratio of the NMR signal over a larger region is to digitally add the post processed signals derived from more than one coil; each sensitive to the precessing nuclei within overlapping volumes. If two coils' signals are processed and converted into image data separately and then added digitally, one can obtain an increase in S/N ratio (SNR) within the larger volume. Separate amplifiers, analog-to-digital converters, sample-and-hold circuits, computer storage, and image processor channels represent an alternative configuration for processing the two signals in lieu of a single quadrature combiner. A system of four channels whose signals are derived from an array of four coils is described in U.S. Pat. No. 4,825,162. The primary advantage of this system is that one obtains the signal-to-noise performance of smaller surface coils over a larger geometric region corresponding to increased anatomical coverage.
Yet another method of further improving the SNR is to combine the effective gains of both quadrature coils with those of multiple channel or array systems. Such a system of quadrature arrays is comprised of two sets of linear coils, each element in each set having a phase component orthogonal to the phase component of each element of the sister set. Then, the signals are combined such that each linear signal is paired with its co-volume-sharing paired linear coil signal with the appropriate 90 degree phase shift to yield the quadrature gain in each element pair volume. This coil system is taught in U.S. Pat. No. 5,430,378 ('378 patent) entitled “NMR Quadrature Detection Array”.
Limitations exists with the aforementioned configuration of quadrature coils; that being that they are not laid out to provide optimal volumetric coverage—that is sensitivity from more than one side of the patient. Due to the fact that they are described as being positioned along one aspect of the patient; the sensitivity profiles imparted are assymetrical to the patient. Two patents referenced have made incremental improvements in volumetric signal homogeneity by creating linear arrays that are positioned above (superior) and below (inferior) the patient. Both U.S. Pat. No. 5,548,218 (Lu) and U.S. Pat. No. 6,624,633 B1 (Zou) employ linear arrays of three or more saddle or butterfly elements posterior to the patient and arrays of three or more single loop elements anterior to the patient. Signals from each anterior and posterior element, arranged opposing each other across the patient volume, are then added in quadrature mixers to create quadrature signals from the medial regions where their signals exhibit the same relative signal strength. These configurations are limited as well due to that simple fact that each element of the quadrature pair has substantially different sensitivity (flux) profiles throughout the medial volume due to their positions being on opposite sides of the volume. Quadrature combination of these signals yields a combined signal that is mostly that signal of the saddle or butterfly coil on those coil's sides of the volume and the combined signal that is dominated by the single loop signal on the single loop side of the volume. It is only in the middle of the volume where the saddle and single loop signals are similar magnitude where effective quadrature gain is realized. So, although the aforementioned patents describe volume coil arrays that provide more homogeneous signal quality throughout a volume, they do not yield the SNR performance locally to the quadrature coil sets described in the '378 patent.
The problem of improved volumetric homogeneity without sacrifice of SNR could be solved by arranging quadrature arrays similar as described in the '378 patent on more than one side of a patient, and of course using care to ensure that element sizes, orientations, and resulting signal phases were such that each element's signals were not destructive to one another. This solution then brings about a two piece coil set that is relatively easy to position about the patient's torso such as presented by U.S. Pat. No. 6,650,926 B1 (Chan et. al.). This particular patent is based upon creating a series of quadrature paired elements overlapping in the Z-direction (long axis of the patient) and held within position of one another by a semi-rigid spline, or spline that is hinged near its center to facilitate some flexibility along the Z-direction. Flexible components of the antenna elements protrude from the central spline and partially wrap about the subject. Opposing anterior and posterior rigid spline coil sets facilitate wrapping from both sides of a patient and creating a uniform quadrature detection volume. This design is limited in the number of elements, has limited flexibility from a generally planar configuration, and doesn't address optimization of multiple elements on such a flexible form.
In the case of the extremities, in contrast to the potentially much larger diameter torso, a different solution is possible that brings the convenience of a singular coil structure versus opposing two-part structures. One solution, presented in U.S. Pat. No. 6,438,402 B1 (Hashoian) is to wrap larger resonating elements about both legs and lower torso with a series of overlapping elements.
Another solution, considering the smaller diameters and lengths of extremities versus the human form, is to place a singular structure of reducing diameter about a single extremity, and with sufficient length and number of elements to optimize the SNR throughout the entire length of the extremity or body part. This concept may appear similar to that of U.S. Pat. No. 6,438,402 B1 (Hashoian), but there exists significant conceptual differences.
First, Hashoian teaches the creation of quadrature pairs of elements within a singular cylindrical wrap, then teaches that multiple wraps can be added in an overlapping fashion; hence creating an array providing considerable longitudinal coverage. For proper tuning to be maintained, the relative flexible antenna structures must maintain their relative shape and position relative to one another; a difficult feat with this mechanical design as there is little that will keep the adjacent structures with the proper critical overlap for the requisite inductive isolation. If the isolation or tuning of an individual element is perturbed due to improper flexing or placement, the uniformity of the exam will be seriously compromised. Secondly, the design requires latching each and every wrapped element separately; a cumbersome task and time sensitive task considering the need for the patient to remain motionless throughout the entire exam and the nature of throughput requirements in MRI.
Thirdly, Hashoian does not teach how to create an array of more than two adjacent quadrature elements; hence compromising the possible SNR compared to an array with all quadrature elements.
Finally, Hashoian does not address optimization of the element size, number, tuning stability or isolation.
Similar antenna geometries of Hashoian are incorporated by Szumowski in U.S. Pat. No. 6,137,291 and Vij in U.S. Pat. No. 6,498,489 B1; however, Szumowski and Vij teach rigid, separable saddle coil pairs or helmholtz pairs versus the flexible elements that Hashoian uses. Both Szumowski and Vij utilize the similar concept of reducing cylindrical diameter to ensure closer coupling to the anatomies in question but neither teaches quadrature elements “surrounding” the anatomies along the entire length of the anatomies.
U.S. Pat. No. 5,435,302 discloses flexible antennas wherein a singular resonator is constructed on a flexible substrate. This patent divulges a method of mounting thin conductors of a single resonator on a preshaped pseudo-flexible form for scanning one unique patient anatomy.
Although U.S. Pat. No. 5,594,339 also teaches some construction methods for creating a flexible coil substrate, it is restrictive in practice as the sheet plastic layers flex in an arch tangential along one axis only. Neither of these two previously mentioned patents teaches coil arrays, or quadrature arrays or the methods required for making such arrays operable (ie. tuning stability, maintaining isolation, and flexing in three dimensions) in a highly flexible environment.
Two more recent patents address the need for multiple elements on shaped forms with contours along all three axes such as a helmet-like coil form or shoulder-torso form. U.S. Pat. No. 6,084,411 ('411 patent) and U.S. Pat. No. 7,663,367 B2 describe the construction of a 3-dimensional form-fitting rigid or fixed position substrate on which independent coil resonators are attached ('411 patent) or manufactured as a traditional overlapping or non-overlapping (for parallel imaging sequence performance optimization) multi-element array as is taught in the scientific literature (many such articles in the Journal of Magnetic Resonance Imaging). Neither patent anticipates a highly flexible antenna array that maintains proper operational capability while being flexed in infinite positions.